Commercial Buildings: Towards Hybrid Framing

So how does this combination of factors impact on the design of structural framing of workplaces? There is a definite struggle between the requirement to create flexibility whilst also delivering a product that creates intimacy between user and environment.

For the previous thirty years, Central London commercial developments were predominately 8 to 10 storeys of long-span steel frames with composite concrete floor plates cast on permanent metal decks. The design mantra was to push for the largest possible floor plates with a minimum number of internal columns, whilst maximizing floor-to-ceiling heights.

More recently we have seen a move towards mass timber office construction. This has been driven by a desire to control the embodied carbon of the building whilst providing a higher-quality internal environment. At the BskyB “Believe in Better Building” we worked with architects Arup Associates to develop a 4 storey mass timber office building with Glued-Laminated Timber (GLT) beams and columns and Cross Laminated Timber (CLT) floor plates. Stability was provided by CLT core walls.

The conclusions were surprising. Of all the options considered, the clear winner replaced all the secondary cellular steel beams with a composite CLT / GLT Rib-deck floor plate but kept steel framing to columns, primary beams and façade beams. The reinforced concrete core was replaced with CLT walls. The upfront costs were comparable to the existing scheme but the erection programme was cut as the core and floor plates could be erected concurrently. In terms of upfront embodied carbon (excluding consideration of carbon sequestered), the original scheme weighed in at 330 kgCO2e/m2. The winning option measured at 150 kgCO2e/m2 (a 55% reduction in A1-A3 embodied carbon). If sequestration of the timber was taken into account, it could be considered as a negative embodied carbon structural frame with a measure of -82 kgCO2e/m2.